In recent years, the xDSL communication systems including the ADSL (Asymmetric Digital Subscriber Line) communication system, the HDSL (High-bit-rate Digital Subscriber Line) communication system and the SDSL communication system for performing a high-speed digital communication of several mega bits per second using the existing telephone copper cable have been closely watched. The xDSL communication system used for these systems, is called the DMT (Discrete Multi-Tone) modem system. This system is standardized in T1.413, etc. of ANSI.
This digital communication system, especially in the case where the xDSL transmission path and the ISDN transmission path of the half-duplex ISDN communication system are bound together as an aggregated line or otherwise placed adjacently to each other, poses the problem that the xDSL communication through the xDSL transmission path is affected by interference noises from the ISDN transmission path or other lines and decreases in speed. For solving this problem, various devices are introduced.
FIG. 19 shows the interference noises of an ISDN transmission path 2 from a central office (CO) 1, which affect an ADSL transmission path 3 constituting a xDSL transmission path bound with the ISDN transmission path 2 midway as an aggregated line.
When viewed from the ADSL terminal equipment (ATU-R; ADSL transceiver unit, remote terminal end) 4 constituting a communication unit at a terminal of the ADSL communication system, the interference noise transmitted through the ADSL transmission path 3 by the office equipment (ISDN LT) 7 of the ISDN transmission system is called the FEXT (Far-End crossTalk) noise, while the interference noise transmitted through the ADSL transmission path 3 by the terminal equipment (ISDN NT1) 6 of the ISDN transmission system is called the NEXT (Near-End crossTalk) noise. Especially, these noises are transmitted to the ADSL terminal equipment (ATU-R) 4 through the ADSL transmission path 3 which is coupled with the ISDN transmission path 2 midway as an aggregated line.
When viewed from the ADSL office equipment (ATU-C: ADSL transceiver unit, central office end) 5 constituting the office equipment of the ADSL communication system, on the other hand, the result is opposite from the case viewed from the ADSL terminal equipment (ATU-R) 4. In such a case, the interference noise transmitted by the office equipment (ISDN LT) 7 of the ISDN transmission system constitutes the NEXT noise, while the interference noise transmitted by the terminal equipment (ISDN NT1) 6 of the ISDN transmission system makes up the FEXT noise.
In the US ISDN communication system which is full-duplexed, the up and down transmissions are performed at the same time. When viewed from the ADSL terminal equipment (ATU-R) 4, therefore, the NEXT noise generated by the terminal equipment (ISDN NT1) 6 of the ISDN transmission system nearer to the ADSL terminal equipment (ATU-R) 4 is controlling, i.e. has a larger effect.
For this reason, during the training period of the ADSL modem (not shown) installed at the ADSL terminal equipment (ATU-R) 4, the characteristic of the NEXT noise components having a large effect is measured, and the number of transmission bits and the gain of each channel meeting the noise characteristic are determined by bit mapping. Further, in order to improve the transmission characteristics, the coefficients of the time domain equalizer (TEQ) for adaptive equalization in time domain and the frequency domain equalizer (FEQ) for adaptive equalization in frequency domain are converged and determined, so that a set of coefficient tables for NEXT noises are provided for each of TEQ and FEQ.
Although this measure eliminates the problem in the above described digital communication systems, the half-duplex communication system TCM-ISDN employed in Japan as an existing ISDN communication system, in which the up and down data transmission are switched by time division like Ping-Pong, poses the problem. Namely, in the case where the half-duplex transmission path and other transmission path are adjacently placed to each other as an aggregated line or the like, the NEXT noises and the FEXT noises from the half-duplex transmission path have an effect alternately on the communication terminals connected to the other transmission paths adjacent to the half-duplex transmission path.
In the Japanese ADSL system, therefore, a method is proposed in which the bit map is switched in accordance with the FEXT and NEXT sections of the TCM-ISDN interference noises (“G.lite: Proposal for draft of Annex of G.lite”, ITU-T, SG-15, Waikiki, Hi. 29 June-3 Jul. 1998, Temporary Document WH-047).
FIG. 20 shows an outline of a digital communication system using the digital communication equipment employing the method described in the above literature.
In FIG. 20, numeral 11 designates a central office (CO) for controlling the TCM-ISDN communication and the ADSL communication, numeral 12 designates a TCM-ISDN transmission path for the TCM-ISDN communication, numeral 13 designates an ADSL transmission path for the ADSL communication, numeral 14 designates an ADSL terminal equipment (ATU-R; ADSL transceiver unit, remote terminal end) such as a communication modem for performing the ADSL communication with other ADSL communication terminal equipment (not shown) through the ADSL transmission path 13, numeral 15 designates an ADSL office equipment (ATU-C; ADSL transceiver unit, central office end) for controlling the ADSL communication within the central office 11, numeral 16 a TCM-ISDN terminal equipment (TCM-ISDN NT1) such as a communication modem for performing the TCM-ISDN communication with other TCM-ISDN terminal equipment (not shown) through the TCM-ISDN transmission path 12, numeral 17 designates a TCM-ISDN office equipment (TCM-ISDN LT) for controlling the TCM-ISDN communication in the central office 11, and numeral 18 designates a sync controller for synchronizing the communication between the TCM-ISDN office equipment (TCM-ISDN LT) 17 and the ADSL office equipment (ATU-C) 15. The sync controller 18 may alternatively be installed in the TCM-ISDN office equipment (TCM-ISDN LT) 17 or in the ADSL office equipment (ATU-C) 15.
As described above, the interference noise transmitted, through the TCM-ISDN transmission path 12 and the ADSL transmission path 13 adjacently placed to each other as an aggregated line, by the TCM-ISDN office equipment (TCM-ISDN LT) 17 providing a far half-duplex communication system when viewed from the ADSL terminal equipment (ATU-R) 14, as shown in FIG. 20, is called the “FEXT noise”. On the other hand, the interference noise transmitted, through the TCM-ISDN transmission path 12 and the ADSL transmission path 13 adjacently placed to each other as an aggregated line, by the TCM-ISDN terminal equipment (TCM-ISDN NT1) 16 constituting a near half-duplex communication system is called the “NEXT noise”.
When viewed from the ADSL office equipment (ATU-C) 15, on the other hand, the case is opposite to the view from the ADSL terminal equipment (ATU-R) 14. Namely, the interference noise transmitted by the office equipment (ISDN LT) 17 of the ISDN transmission system constituting the near half-duplex communication system is the NEXT noise, while the interference noise transmitted by the terminal equipment (ISDN NT1) 6 of the ISDN transmission system making up a far half-duplex communication system constitutes the FEXT noise.
FIG. 21 shows a functional configuration of a transmission unit or a dedicated transmitter (hereinafter referred to as the transmission system) such as a communication modem of the ADSL office equipment (ATU-C; ADSL transceiver unit, central office end) 15 of the digital communication system. On the other hand, FIG. 22 shows a functional configuration of a receiving unit or a dedicated receiver (hereinafter referred to as the receiving system) such as a communication modem of the ADSL terminal equipment (ATU-R) 14 of the digital communication system.
In FIG. 21, numeral 41 designates a multiplex/sync control, numerals 42, 43 designate cyclic redundancy checks (crc), numerals 44, 45 designate scramble forward error corrections (Scram & FEC), numeral 46 designates an interleave, numerals 47, 48 designate rate convertors, numeral 49 designates tone ordering, numeral 50 designates constellation encoder and gain scaling, numeral 51 designates inverse discrete Fourier transform (IDFT), numeral 52 designates a input parallel/serial buffer, and numeral 53 designates an analog processing D/A converter (DAC).
In FIG. 22, numeral 141 designates an analog processing A/D converter (ADC), numeral 142 designates a time domain equalizer (TEC), numeral 143 designates an input serial/parallel buffer, numeral 144 designates discrete Fourier transform (DFT), numeral 145 designates a frequency domain equalizer (FEQ), numeral 146 designates constellation encoder and gain scaling, numeral 147 designates tone ordering, numerals 148, 149 designate rate convertors, numeral 150 designates deinterleave, numerals 151, 152 designate descramble and forward error correction (Descram&FEC), numerals 153, 154 designate a cyclic redundancy check (crc), and numeral 155 designates multiplex/sync control.
Now, the operation will be explained. To begin with, the operation of the transmission system of the ADSL office equipment (ATU-C) 15 will be explained. In FIG. 21, the transmission data are multiplexed by the multiplex/sync control 41, and have an error detection code added thereto by the cyclic redundancy checks 42, 43, have the FEC code added thereto and subjected to the scramble processing by the scramble forward error corrections 44, 45, sometimes followed by the processing in the interleave 46. After that, the rate is converted by the rate convertors 47, 48, the tone ordering is executed by the tone ordering 49, the constellation data are produced by the constellation encoder and gain scaling 50, the inverse discrete Fourier transform is carried out by the inverse discrete Fourier transform 51, the digital waveform is converted into an analog waveform through the D/A converter, and then the signal is applied through a low-pass filter.
The operation of the receiving system of the ADSL terminal equipment (ATU-R) 14 will now be explained. In FIG. 22, the analog processing A/D converter 141 applies the received signal through a low-pass filter, and converts the analog waveform into a digital waveform through the A/D converter, followed by the time domain adaptive equalization in the time domain equalizer (TEQ) 142.
Then, the data subjected to the time domain adaptive equalization are converted from serial to parallel data by the input serial/parallel buffer 143, subjected to discrete Fourier transform in the discrete Fourier transform (DFT) 144, and then subjected to the frequency domain adaptive equalization by the frequency domain equalizer (FEQ) 145.
The constellation data are reproduced by the constellation encoder and gain scaling 146, converted into the serial data by the tone ordering 147, have the rate converted in the rate convertors 148, 149, subjected to the descramble processing and FEC by the descramble and forward error correction 151, and in some cases, after being deinterleaved by the deinterleave 150, subjected to FEC and descramble processing by the descramble and forward error correction 152. After the processing in the cyclic redundancy check 153, 154, the data are reproduced by the multiplex/sync control 155.
In this process, the sync controller 18 of the central office (CO) 11 synchronizes the transmission timing between the TCM-ISDN office equipment (TCM-ISDN LT) 17 and the ADSL office equipment (ATU-C) 15. Thus, the ADSL terminal equipment (ATU-R) 14 can recognize the timing of generation of the NEXT noise and FEXT noise.
Specifically, the ADSL terminal equipment (ATU-R) 14, by the synchronization between the TCM-ISDN communication and the ADSL communication, determines that the NEXT noise is generated in the received data or the signal received through the ADSL transmission path 13 during a predetermined time when the data are transmitted up the TCM-ISDN transmission path 12 at a known timing. On the other hand, during a predetermined time when the data are transmitted down the TCM-ISDN transmission path 12 at a known timing, the generation of the FEXT noise can be similarly recognized in the data received through the ADSL transmission path 13.
In the Japanese ADSL system, as shown in FIG. 23, the bit maps A and the bit maps B are assigned to the FEXT sections and the NEXT sections, respectively, and in the rate convertors 148, 149 of FIG. 21, more bits are assigned to the FEXT section having a small noise amount, and less bits are assigned to the NEXT section having a large noise. As a result, the transmission rate can be improved as compared with the conventional case in which the bit distribution is determined only by the NEXT section.
FIG. 24 shows the manner in which the data received at uniform rate (64 kbps in the calculation example below) are assigned to the bit maps A and the bit maps B at the time of transmission. First, the data sent in at uniform rate are stored in the form of fixed bits in units of symbols. These data are converted into bits for the bit map A and the bit map B by a rate convertor. An integer multiple is not involved, however, because the interval of the transmitted symbols is 246 μs for the ISDN period of 2.5 ms.
Thus, as shown in FIG. 25, with 34 periods (=345 symbols, 85 ms) as one unit (hyperframe), only the FEXT section in the hyperframe where the symbols are filled up is defined as a bit map A, and the other portions as a bit map B (in the drawing, SS and ISS indicate sync signals). Whether each DMT symbol is associated with the bit map A or the bit map B is determined from the following equations. In the equations below, the DMT symbol No. is assumed to be Ndmt.
.Transmission from ATU-C to ATU-R
S=272×Ndmt mod 2760
if {(S+271<a) or (S>a+b)} then [bit map A symbol]
if {(S+271>=a) and (S<=a+b)} then [bit map B symbol]
where a=1243, and b=1461.
.Transmission from ATU-R to ATU-C
S=272×Ndmt mod 2760
if {(S>a) and (S+271<a+b)} then [bit map A symbol]
if {(S<=a) or (S+271>=a+b)} then [bit map B symbol]
where a=1315, and b=1293.
An example of calculation for determining the bit assignment for the single bit map using only the bit map A is shown below.
.Number of bits of 1 DMT symbol (before rate conversion)
=(transmission rate)×(transmission time)/(total number of symbols (except for ISS (inverse sync symbol) and SS (sync symbol)))
=64 kbps×85 ms/340
=16 bits
.Number of bits of bit map A
=(transmission rate)×(transmission time)/(total number of symbols of bit map A (except for ISS (inverse sync symbol) and SS (side A sync symbol)))
=64 kbps×85 ms/126
=43.175
Thus, the bit map A is assumed to be equal to 44 bits. Also, because of the single bit map (only the bit map A is used), the bit map B is set to zero.
The following is an example of calculation for determining the bit assignment for the dual bit map where both the bit map A and the bit map B are used.
.Number of bits of 1 DMT symbol (before rate conversion)
=(transmission rate)×(transmission time)/(total number of symbols (except for ISS (inverse sync symbol) and SS (sync symbol)))
=64 kbps×85 ms/340
=16 bits
.This calculation example assumes that the number of bits of the bit map B is 3.
Number of bits of bit map A
=((transmission rate)×(transmission time)−(number of bits per symbol of bit map B)×(number of symbols of bit map B (except for ISS (inverse sync symbol) and SS (side A sync symbol)))/(number of symbols of bit map A (except for ISS (inverse sync symbol) and SS (side A sync symbol)))
=(64 kbps×85 ms −3×214)/126
=38.079 bits
Thus, the bit map A has 39 bits. When changing the bit distribution by a rate convertor in this way, the data are output after being stored to some degree in the rate convertor at the transmitting end or the receiving end, and therefore a delay time occurs in the rate convertor. Further, with a single bit map, the transmission data are assigned to the bit map A as fully as possible by hyperframe, and therefore the data of some period may be assigned to the bit map A of subsequent periods. Thus, such data causes a further delay time. Also in the case of the dual bit map, bits are assigned to the bit map A and the bit map B of the hyperframe as fully as possible, and therefore the data of a given period may be assigned to the subsequent periods, with the result that a further delay time is caused for such data. In this conventional system, the problem of an excessive delay is posed.
Accordingly, the object of the present invention, which has been developed to solve this problem, is to provide a communication system and a communication method capable of suppressing the delay.